1. Field
This disclosure relates to a negative electrode for a rechargeable lithium battery, a rechargeable lithium battery including the same, and a method of preparing a rechargeable lithium battery.
2. Description of the Related Art
Recently, due to reductions in size and weight of portable electronic equipment, there has been a need to develop suitable batteries for the portable electronic equipment that has both high performance and large capacity. Here, a rechargeable lithium battery includes a positive electrode (including a positive active material capable of intercalating/deintercalating lithium ions), a negative electrode (including a negative active material capable of intercalating/deintercalating lithium ions), and an electrolyte.
When the negative active material is amorphous carbon and graphite, lithium ions are intercalated into carbon of a negative electrode and form an SEI (solid electrolyte interface) film on the surface thereof. The SEI passivation film can be referred to as a membrane for passing not electrons but lithium ions and helps the negative electrode work properly.
However, as the SEI passivation film becomes thicker, an insulation layer becomes thicker, thereby increasing interface resistance. In addition, when the SEI passivation film is too porous and not firm, the SEI passivation film may be broken by a physical impact due to contraction/expansion of negative active material particles during the charge and discharge, thereby exposing the surface of a negative active material layer to an electrolyte solution.
Herein, the exposed surface of the negative active material layer results in a reduction reaction with the electrolyte and forms a new SEI passivation film thereon. Since a high-capacity battery fully charges and fully discharges relatively slowly at a low C-rate up to a range of about 3V to about 4.3V, an active material may not undergo abrupt lattice changes. In addition, the active material has a volume change at a very low rate.
Accordingly, an additive for preventing decomposition of the electrolyte, for example, vinylene carbonate (VC), fluoroethylene carbonate (FEC), and the like should be added to the active material to form a thin SEI passivation film and thus, suppressing the decomposition of the electrolyte. However, unlike the high-capacity battery, the surface of an active material layer needs to be newly designed because of an increasing request of high rate output/high rate input and the like from consumers.
A conventional SEI passivation film may suppress a positive lithium source but has a problem of being easily deteriorated by its abrupt volume contraction/expansion on the surface of negative active material particles due to high-rate input and output, which keeps exposing the surface of the negative active material layer. Accordingly, the active material particles have sharply increased interface resistances on their surfaces and thus, deteriorating their cycle-life as well as generating undesired gas.